The word "benison" is from Middle English and means "spoken prayer," as in a benediction or blessing. The modern translation of this speech in eNotes etexts reads:

God's blessings go with you; and with thoseThat would make a good situation out of a bad one and friends of foes! (Act 2, scene 4)

When the scene opens Ross and a character called Old Man are standing outside Macbeth's castle and talking about all the strange things that have been happening. The Old Man says this line as they part, so it could be that this was a typical farewell people said to each other. It could also be a prayer for protection from all the "signs and portents." The eNotes commentary states: "The blessing is a counterpoint to the evils related at the beginning of the scene. It will not, however, be sufficient to neutralise the damage that Macbeth has done."